Editing Endometrium

{{Short description|Inner mucous membrane of the mammalian uterus}}
{{Infobox anatomy
| Name        = Endometrium
| Latin       = tunica mucosa uteri
| Image       = Basic_Female_Reproductive_System_(English).svg
| Caption     = Uterus and [[fallopian tubes]] (uterine tubes). (Endometrium labeled at center right.)
| Width       = 
| Image2      =Proliferative phase endometrium -- high mag.jpg 
| Caption2    =Endometrium in the proliferative phase 
| Precursor   = 
| System      = 
| Part_of     = [[Uterus]]
| Artery      = 
| Vein        = 
| Nerve       = 
| Lymph       = 
}}

The '''endometrium''' is the inner [[epithelium|epithelial]] layer, along with its [[mucous membrane]], of the [[mammal]]ian [[uterus]].  It has a basal layer and a functional layer: the basal layer contains [[stem cell]]s which regenerate the functional layer.<ref name="Gargett">{{cite journal|last1=Gargett|first1=C.E.|last2=Schwab|first2=K.E.|last3=Zillwood|first3=R.M.|last4=Nguyen|first4=H.P.|last5=Wu|first5=D.|title=Isolation and culture of epithelial progenitors and mesenchymal stem cells from human endometrium.|journal=Biology of Reproduction|date=June 2009|volume=80|issue=6|language=en|pages=1136–1145|doi=10.1095/biolreprod.108.075226|pmid=19228591|pmc=2849811}}</ref> The functional layer thickens and then is shed during [[menstruation]] in humans and some other mammals, including other [[ape]]s, [[Old World monkey]]s, some species of [[bat]], the [[elephant shrew]]<ref name="Emera">{{cite journal|last1=Emera|first1=D|last2=Romero|first2=R|last3=Wagner|first3=G|title=The evolution of menstruation: a new model for genetic assimilation: explaining molecular origins of maternal responses to fetal invasiveness.|journal=BioEssays|date=January 2012|volume=34|issue=1|pages=26–35|doi=10.1002/bies.201100099|pmid=22057551|pmc=3528014}}</ref> and the [[Cairo spiny mouse]].<ref>{{Cite journal|last1=Bellofiore|first1=N.|last2=Ellery|first2=S.|last3=Mamrot|first3=J.|last4=Walker|first4=D.|last5=Temple-Smith|first5=P.|last6=Dickinson|first6=H.|date=2016-06-03|title=First evidence of a menstruating rodent: the spiny mouse (Acomys cahirinus)|url=https://www.biorxiv.org/content/10.1101/056895v1|journal=bioRxiv|volume=216|issue=1|language=en|pages=40.e1–40.e11|doi=10.1101/056895|pmid=27503621|s2cid=196624853}}</ref> In most other mammals, the endometrium is reabsorbed in the [[estrous cycle]]. During [[pregnancy]], the glands and [[blood vessel]]s in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the [[placenta]], which supplies [[oxygen]] and nutrition to the [[embryo]] and [[fetus]].<ref name=histology>[http://www.lab.anhb.uwa.edu.au/mb140/CorePages/FemaleRepro/FemaleRepro.htm Blue Histology - Female Reproductive System] {{Webarchive|url=https://web.archive.org/web/20070221174122/http://www.lab.anhb.uwa.edu.au/mb140/CorePages/FemaleRepro/FemaleRepro.htm |date=2007-02-21 }}. School of Anatomy and Human Biology&nbsp;— The University of Western Australia Accessed 20061228 20:35</ref><ref name=Guyton2006>{{cite book | title =Textbook of Medical Physiology | chapter = Chapter 81 Female Physiology Before Pregnancy and Female Hormones |pages =  1018ff  | edition = 11th |veditors= Guyton AC, Hall JE|publisher = Elsevier Saunders | year = 2006 | isbn = 9780721602400}}</ref> The speculated presence of an endometrial microbiota<ref name="FranasiakScott2015">{{cite journal|last1=Franasiak|first1=Jason M.|last2=Scott|first2=Richard T.|title=Reproductive tract microbiome in assisted reproductive technologies|journal=Fertility and Sterility|volume=104|issue=6|year=2015|pages=1364–1371|issn=0015-0282|doi=10.1016/j.fertnstert.2015.10.012|pmid=26597628 |doi-access=free}}</ref>
has been argued against.<ref name="Baker">{{cite journal|last1=Baker|first1=JM|last2=Chase|first2=DM|last3=Herbst-Kralovetz|first3=MM|title=Uterine Microbiota: Residents, Tourists, or Invaders?|journal=Frontiers in Immunology|date=2018|volume=9|pages=208|doi=10.3389/fimmu.2018.00208|pmid=29552006|pmc=5840171|doi-access=free}}</ref><ref name="Microbiome">{{cite journal|last1=Perez-Muñoz|first1=ME|last2=Arrieta|first2=MC|last3=Ramer-Tait|first3=AE|last4=Walter|first4=J|title=A critical assessment of the "sterile womb" and "in utero colonization" hypotheses: implications for research on the pioneer infant microbiome.|journal=Microbiome|date=28 April 2017|volume=5|issue=1|pages=48|doi=10.1186/s40168-017-0268-4|pmid=28454555|pmc=5410102 |doi-access=free }}</ref>

==Structure==
[[File:Histology of normal simple columnar epithelium of the endometrium.jpg|thumb|Histology of the most superficial layer of the endometrium, consisting of a simple columnar epithelium. [[H&E stain]]]]
[[File:Endometrium ocp use3.jpg|thumb|right|High magnification [[micrograph]] of [[decidualization|decidualized]] endometrium due to exogenous [[progesterone]] ([[oral contraceptive pill]]). [[H&E stain]]]]
[[File:Endometrium ocp use0.jpg|thumb|right|Low magnification micrograph of decidualized endometrium. H&E stain]]

The endometrium consists of a single layer of [[columnar epithelium]] plus the [[Stroma (animal tissue)|stroma]] on which it rests. The stroma is a layer of [[connective tissue]] that varies in thickness according to [[hormonal]] influences. In the [[uterus]], simple [[tubular gland]]s reach from the endometrial surface through to the base of the stroma, which also carries a rich blood supply provided by the [[spiral artery|spiral arteries]]. In women of reproductive age, two layers of endometrium can be distinguished. These two layers occur only in the endometrium lining the cavity of the uterus, and not in the lining of the [[fallopian tube]]s where a potentially life-threatening [[ectopic pregnancy]] may occur nearby.<ref name=histology/><ref name=Guyton2006/>

* The ''functional layer'' is adjacent to the uterine cavity. This layer is built up after the end of menstruation during the first part of the previous [[menstrual cycle]]. Proliferation is induced by [[estrogen]] (follicular phase of menstrual cycle), and later changes in this layer are engendered by progesterone from the [[corpus luteum]] (luteal phase). It is adapted to provide an optimum environment for the [[Implantation (embryology)|implantation ]] and growth of the [[embryo]]. This layer is completely shed during [[menstruation]].
* The ''basal layer'', adjacent to the [[myometrium]] and below the functional layer, is not shed at any time during the menstrual cycle. It contains stem cells that regenerate the functional layer,<ref name="Gargett"/> which develops on top of it.
In the absence of progesterone, the arteries supplying blood to the functional layer constrict, so that cells in that layer become [[ischaemic]] and die, leading to [[menstruation]].

It is possible to identify the phase of the menstrual cycle by reference to either the [[Menstrual cycle#Ovarian cycle|ovarian cycle]] or the [[Menstrual cycle#Uterine cycle|uterine cycle]] by observing microscopic differences at each phase—for example in the ovarian cycle:

{| class="wikitable"
! Phase !! Days !! Thickness !! Epithelium
 |-
 | Menstrual phase || 1–5 || Thin || Absent
 |-
 | [[Follicular phase]] || 5–14 || Intermediate || Columnar
 |-
 | [[Luteal phase]] || 15–27 || Thick || Columnar. Also visible are [[arcuate vessels of uterus]]
 |-
 | [[Ischemic phase]] || 27–28 ||  || Columnar. Also visible are arcuate vessels of uterus
|}

===Gene and protein expression===
{{Further |Bioinformatics#Gene and protein expression}}

About 20,000 protein coding genes are expressed in human cells and some 70% of these genes are expressed in the normal endometrium.<ref>{{Cite web|url=https://www.proteinatlas.org/humanproteome/endometrium|title=The human proteome in endometrium - The Human Protein Atlas|website=www.proteinatlas.org|access-date=2017-09-25}}</ref><ref>{{Cite journal|last1=Uhlén|first1=Mathias|last2=Fagerberg|first2=Linn|last3=Hallström|first3=Björn M.|last4=Lindskog|first4=Cecilia|last5=Oksvold|first5=Per|last6=Mardinoglu|first6=Adil|last7=Sivertsson|first7=Åsa|last8=Kampf|first8=Caroline|last9=Sjöstedt|first9=Evelina|date=2015-01-23|title=Tissue-based map of the human proteome|journal=Science|language=en|volume=347|issue=6220|pages=1260419|doi=10.1126/science.1260419|issn=0036-8075|pmid=25613900|s2cid=802377}}</ref> Just over 100 of these genes are more specifically expressed in the endometrium with only a handful genes being highly endometrium specific. The corresponding specific proteins are expressed in the glandular and stromal cells of the endometrial mucosa. The expression of many of these proteins vary depending on the menstrual cycle, for example the [[progesterone receptor]] and [[thyrotropin-releasing hormone]] both expressed in the proliferative phase, and [[PAEP]] expressed in the secretory phase. Other proteins such as the [[HOXA11|HOX11]] protein that is required for female fertility, is expressed in endometrial stroma cells throughout the menstrual cycle. Certain specific proteins such as the [[Estrogen receptor alpha|estrogen receptor]] are also expressed in other types of female tissue types, such as the [[cervix]], [[fallopian tube]]s, [[Ovary|ovaries]] and [[breast]].<ref>{{Cite journal|last1=Zieba|first1=Agata|last2=Sjöstedt|first2=Evelina|last3=Olovsson|first3=Matts|last4=Fagerberg|first4=Linn|last5=Hallström|first5=Björn M.|last6=Oskarsson|first6=Linda|last7=Edlund|first7=Karolina|last8=Tolf|first8=Anna|last9=Uhlen|first9=Mathias|date=2015-10-21|title=The Human Endometrium-Specific Proteome Defined by Transcriptomics and Antibody-Based Profiling|journal=OMICS: A Journal of Integrative Biology|volume=19|issue=11|pages=659–668|doi=10.1089/omi.2015.0115|pmid=26488136}}</ref>

===Microbiome speculation===
The uterus and endometrium was for a long time thought to be sterile. The [[cervical mucus plug|cervical plug]] of mucosa was seen to prevent the entry of any [[microorganism]]s ascending from the vagina. In the 1980s this view was challenged when it was shown that uterine infections could arise from weaknesses in the barrier of the cervical plug. Organisms from the vaginal microbiota could enter the uterus during [[uterine contractions]] in the menstrual cycle. Further studies sought to identify microbiota specific to the uterus which would be of help in identifying cases of unsuccessful [[IVF]] and miscarriages. Their findings were seen to be unreliable due to the possibility of cross-contamination in the sampling procedures used. The well-documented presence of ''[[Lactobacillus]]'' species, for example, was easily explained by an increase in the vaginal population being able to seep into the cervical mucous.<ref name="Baker"/> Another study highlighted the flaws of the earlier studies including cross-contamination. It was also argued that the evidence from studies using germ-free offspring of [[axenic]] animals (germ-free) clearly showed the sterility of the uterus. The authors concluded that in light of these findings there was no existence of a [[microbiome]].<ref name="Microbiome"/>

The normal dominance of Lactobacilli in the vagina is seen as a marker for vaginal health. However, in the uterus this much lower population is seen as invasive in a closed environment that is highly regulated by female sex hormones, and that could have unwanted consequences. In studies of [[endometriosis]] ''Lactobacillus'' is not the dominant type and there are higher levels of ''[[Streptococcus]]'' and ''[[Staphylococcus]]'' species. Half of the cases of [[bacterial vaginitis]] showed a polymicrobial [[biofilm]] attached to the endometrium.<ref name="Baker"/>

==Function==
The endometrium is the innermost lining layer of the [[uterus]], and functions to prevent adhesions between the opposed walls of the [[myometrium]], thereby maintaining the patency of the uterine cavity.<ref name="proteinatlas2">{{cite web |title=Dictionary - Normal: Endometrium - The Human Protein Atlas |url=https://www.proteinatlas.org/learn/dictionary/normal/endometrium |website=www.proteinatlas.org |access-date=28 December 2022}}</ref> During the [[menstrual cycle]] or [[estrous cycle]], the endometrium grows to a thick, blood vessel-rich, glandular tissue layer. This represents an optimal environment for the [[implantation (human embryo)|implantation]] of a [[blastocyst]] upon its arrival in the uterus. The endometrium is central, echogenic (detectable using ultrasound scanners), and has an average thickness of 6.7&nbsp;mm.

During [[pregnancy]], the glands and [[blood vessel]]s in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the [[placenta]], which supplies [[oxygen]] and nutrition to the [[embryo]] and [[fetus]].

===Cycle===
The functional layer of the endometrial lining undergoes cyclic regeneration from stem cells in the basal layer.<ref name="Gargett"/> Humans, apes, and some other species display the [[menstrual cycle]], whereas most other mammals are subject to an [[estrous cycle]].<ref name="Emera"/> In both cases, the endometrium initially proliferates under the influence of [[estrogen]]. However, once [[ovulation]] occurs, the ovary (specifically the corpus luteum) will produce much larger amounts of [[progesterone]].  This changes the proliferative pattern of the endometrium to a secretory lining. Eventually, the secretory lining provides a hospitable environment for one or more blastocysts.

Upon fertilization, the egg may implant into the uterine wall and provide feedback to the body with [[human chorionic gonadotropin]] (hCG). hCG provides continued feedback throughout pregnancy by maintaining the corpus luteum, which will continue its role of releasing progesterone and estrogen. In case of implantation, the endometrial lining remains as ''[[decidua]]''. The decidua becomes part of the placenta; it provides support and protection for the gestation.

Without implantation of a fertilized egg, the endometrial lining is either reabsorbed (estrous cycle) or shed (menstrual cycle). In the latter case, the process of shedding involves the breaking down of the lining, the tearing of small connective blood vessels, and the loss of the tissue and blood that had constituted it through the [[vagina]]. The entire process occurs over a period of several days. Menstruation may be accompanied by a series of uterine contractions; these help expel the menstrual endometrium.

If there is inadequate stimulation of the lining, due to lack of hormones, the endometrium remains thin and inactive. In humans, this will result in [[amenorrhea]], or the absence of a menstrual period. After [[menopause]], the lining is often described as being atrophic.  In contrast, endometrium that is chronically exposed to estrogens, but not to progesterone, may become [[hyperplastic]]. Long-term use of [[oral contraceptives]] with highly potent [[progestin]]s can also induce endometrial [[atrophy]].<ref>{{Cite journal | last1 = Deligdisch | first1 = L. | title = Effects of hormone therapy on the endometrium | journal = Modern Pathology | volume = 6 | issue = 1 | pages = 94–106 | year = 1993 | pmid = 8426860 }}</ref><ref>William's Gynecology, McGraw 2008, Chapter 8, Abnormal Uterine Bleeding</ref>

In humans, the cycle of building and shedding the endometrial lining lasts an average of 28 days. The endometrium develops at different rates in different mammals. Various factors including the seasons, climate, and stress can affect its development. The endometrium itself produces certain [[hormone]]s at different stages of the cycle and this affects other parts of the [[reproductive system]].

==Diseases related with endometrium==
[[File:Endometrial histopathologies and cytopathologies.jpg|thumb|300px|Histopathologic and cytopathologic images.<br />(A) proliferative endometrium (Left: HE × 400) and proliferative endometrial cells (Right: HE × 100)<br />(B) secretory endometrium (Left: HE × 10) and secretory endometrial cells (Right: HE × 10)<br />(C) atrophic endometrium (Left: HE × 10) and atrophic endometrial cells (Right: HE × 10)<br />(D) mixed endometrium (Left: HE × 10) and mixed endometrial cells (Right: HE × 10)<br />(E): endometrial atypical hyperplasia (Left: HE × 10) and endometrial atypical cells (Right: HE × 200)<br />(F) endometrial carcinoma (Left: HE × 400) and endometrial cancer cells (Right: HE × 400).]]

[[Chorionic tissue]] can result in marked endometrial changes, known as an [[Arias-Stella reaction]], that have an appearance similar to [[cancer]].<ref name=pmid11756756>{{Cite journal  | last1 = Arias-Stella | first1 = J. | title = The Arias-Stella reaction: facts and fancies four decades after. | journal = Adv Anat Pathol | volume = 9 | issue = 1 | pages = 12–23 |date=Jan 2002 | doi =  10.1097/00125480-200201000-00003| pmid = 11756756 | s2cid = 26249687 }}</ref>  Historically, this change was diagnosed as [[endometrial cancer]] and it is important only in so far as it should not be misdiagnosed as cancer.

* [[Adenomyosis]] is the growth of the endometrium into the muscle layer of the uterus (the [[myometrium]]).
* [[Endometriosis]] is the growth of tissue similar to the endometrium, outside the uterus.<ref name=Lagana2019>{{Cite journal|last1=Laganà|first1=AS|last2=Garzon|first2=S|last3=Götte|first3=M|title=The Pathogenesis of Endometriosis: Molecular and Cell Biology Insights|journal=International Journal of Molecular Sciences|volume=20|issue=22|page=5615|date=10 Nov 2019|doi=10.3390/ijms20225615|pmid =31717614|pmc=6888544 |doi-access=free }}</ref>
* [[Endometrial hyperplasia]]
* [[Endometrial cancer]] is the most common [[cancer]] of the human female genital tract.
* [[Asherman's syndrome]], also known as intrauterine [[adhesions]], occurs when the basal layer of the endometrium is damaged by instrumentation (e.g., [[Dilation and curettage|D&C]]) or infection (e.g., endometrial [[tuberculosis]]) resulting in endometrial sclerosis and adhesion formation partially or completely obliterating the uterine cavity.

Thin endometrium may be defined as an endometrial thickness of less than 8&nbsp;mm. It usually occurs after [[menopause]]. Treatments that can improve endometrial thickness include [[Vitamin E]], [[L-arginine]] and [[sildenafil citrate]].<ref>{{cite journal |vauthors=Takasaki A, Tamura H, Miwa I, Taketani T, Shimamura K, Sugino N |title=Endometrial growth and uterine blood flow: a pilot study for improving endometrial thickness in the patients with a thin endometrium |journal=Fertil. Steril. |volume=93 |issue=6 |pages=1851–8 |date=April 2010 |pmid=19200982 |doi=10.1016/j.fertnstert.2008.12.062 |doi-access=free }}</ref>

[[Gene expression profiling]] using [[cDNA microarray]] can be used for the diagnosis of endometrial disorders.<ref>{{Cite journal| volume = 94| journal = Fertility and Sterility| title = Genome-based expression profiling as a single standardized microarray platform for the diagnosis of endometrial disorder: an array of 126-gene model | first6 = C.| issue = 1| pages = 114–119| doi = 10.1016/j.fertnstert.2009.01.130| pmid = 19328470| year = 2010| last6 = Lee | first5 = C. | first2 = I.| last2 = Chen | first1 = L.| last3 = Chen | first3 = M.| last5 = Wang | first4 = H.| last4 = Yan| last1 = Tseng| doi-access = free}}</ref>
The [[European Menopause and Andropause Society]] (EMAS) released Guidelines with detailed information to assess the endometrium.<ref>{{cite journal |vauthors=Dreisler E, Poulsen LG, Antonsen SL, Ceausu I, Depypere H, Erel CT, Lambrinoudaki I, Pérez-López FR, Simoncini T, Tremollieres F, Rees M, Ulrich LG |title=EMAS clinical guide: Assessment of the endometrium in peri and postmenopausal women |journal=Maturita |volume=75 |issue= 2|pages= 181–90|year=2013 |pmid=23619009 |doi= 10.1016/j.maturitas.2013.03.011}}</ref>

===Embryo transfer===
{{Anchor|Triple-line}}

An endometrial thickness (EMT) of less than 7&nbsp;mm decreases the pregnancy rate in [[in vitro fertilization]] by an [[odds ratio]] of approximately 0.4 compared to an EMT of over 7&nbsp;mm. However, such low thickness rarely occurs, and any routine use of this parameter is regarded as not justified. The optimal endometrial thickness is 10mm. Nevertheless, in human a perfect synchrony is not necessary; if the endometrium is not ready to receive the embryo an ectopic pregnancy may occur. This consist of the implantation of the blast outside the uterus, which can be extremely dangerous.<ref name="KasiusSmit2014">{{cite journal|last1=Kasius|first1=A.|last2=Smit|first2=J. G.|last3=Torrance|first3=H. L.|last4=Eijkemans|first4=M. J. C.|last5=Mol|first5=B. W.|last6=Opmeer|first6=B. C.|last7=Broekmans|first7=F. J. M.|title=Endometrial thickness and pregnancy rates after IVF: a systematic review and meta-analysis|journal=Human Reproduction Update|volume=20|issue=4|year=2014|pages=530–541|issn=1355-4786|doi=10.1093/humupd/dmu011|pmid=24664156|doi-access=free}}</ref>
[[File:Triple-line endometrium.jpg|thumb|''Triple-line'' endometrium measuring 7mm.]]
Observation of the endometrium by [[transvaginal ultrasonography]] is used when administering [[fertility medication]], such as in [[in vitro fertilization]]. At the time of [[embryo transfer]], it is favorable to have an endometrium of a thickness of between 7 and 14 [[millimeter|mm]] with a ''triple-line'' configuration,<ref name="ZhaoZhang2012">{{cite journal|last1=Zhao|first1=Jing|last2=Zhang|first2=Qiong|last3=Li|first3=Yanping|title=The effect of endometrial thickness and pattern measured by ultrasonography on pregnancy outcomes during IVF-ET cycles|journal=Reproductive Biology and Endocrinology|volume=10|issue=1|year=2012|pages=100|issn=1477-7827|doi=10.1186/1477-7827-10-100|pmid=23190428|pmc=3551825 |doi-access=free }}</ref> which means that the endometrium contains a [[hyperechoic]] (usually displayed as light) line in the middle surrounded by two more [[hypoechoic]] (darker) lines. A ''triple-line'' endometrium reflects the separation of the basal layer and the functional layer, and is also observed in the periovulatory period secondary to rising [[estradiol]] levels, and disappears after ovulation.<ref name="BaerwaldPierson2004">{{cite journal|last1=Baerwald|first1=A. R.|last2=Pierson|first2=R. A.|title=Endometrial development in association with ovarian follicular waves during the menstrual cycle|journal=Ultrasound in Obstetrics and Gynecology|volume=24|issue=4|year=2004|pages=453–460|issn=0960-7692|doi=10.1002/uog.1123|pmid=15343603|pmc=2891966}}</ref>

Endometrial thickness is also associated with live births in IVF. The live birth rate in a normal endometrium is halved when the thickness is <5mm.<ref>1. Gallos, I. D. et al. Optimal endometrial thickness to maximize live births and minimize pregnancy losses: Analysis of 25,767 fresh embryo transfers. Reprod. Biomed. Online 37, 542–548 (2018).</ref>

==Endometrial protection==
Estrogens stimulate endometrial [[cell_proliferation|proliferation]] and [[carcinogenesis]].<ref name="pmid20870686">{{cite journal | vauthors = Mueck AO, Seeger H, Rabe T | title = Hormonal contraception and risk of endometrial cancer: a systematic review | journal = Endocr Relat Cancer | volume = 17 | issue = 4 | pages = R263–71 | date = December 2010 | pmid = 20870686 | doi = 10.1677/ERC-10-0076 | url = | doi-access = free }}</ref><ref name="pmid16112947">{{cite journal | vauthors = Kuhl H | title = Pharmacology of estrogens and progestogens: influence of different routes of administration | journal = Climacteric | volume = 8 | issue = Suppl 1| pages = 3–63 | date = August 2005 | pmid = 16112947 | doi = 10.1080/13697130500148875 | s2cid = 24616324 | url = }}</ref><ref name="pmid34841960">{{cite journal | vauthors = Hamoda H | title = British Menopause Society tools for clinicians: Progestogens and endometrial protection | journal = Post Reprod Health | volume = 28 | issue = 1 | pages = 40–46 | date = March 2022 | pmid = 34841960 | doi = 10.1177/20533691211058030 | s2cid = 244749616 | url = }}</ref> Conversely, progestogens inhibit endometrial proliferation and carcinogenesis caused by estrogens and stimulate [[cellular differentiation|differentiation]] of the endometrium into [[decidua]], which is termed [[endometrial transformation]] or decidualization.<ref name="pmid20870686" /><ref name="pmid16112947" /><ref name="pmid34841960" /> This is mediated by the progestogenic and functional [[antiestrogen]]ic effects of progestogens in this tissue.<ref name="pmid16112947" /> These effects of progestogens and their protection against [[endometrial hyperplasia]] and [[endometrial cancer]] caused by estrogens is referred to as ''endometrial protection''.<ref name="pmid20870686" /><ref name="pmid16112947" /><ref name="pmid34841960" /> As a result of endometrial protection, endometrial cells can undergo [[Epithelial–mesenchymal transition|epithelial-mesenchymal transition]] and be detected in menstrual fluid.<ref>{{Cite journal |last1=Kovina |first1=M.V. |last2=Krasheninnikov |first2=M.E. |last3=Dyuzheva |first3=T.G. |last4=Danilevsky |first4=M.I. |last5=Klabukov |first5=I.D. |last6=Balyasin |first6=M.V. |last7=Chivilgina |first7=O.K. |last8=Lyundup |first8=A.V. |date=2018 |title=Human endometrial stem cells: High-yield isolation and characterization |url=https://linkinghub.elsevier.com/retrieve/pii/S1465324918300045 |journal=Cytotherapy |language=English |volume=20 |issue=3 |pages=361–374 |doi=10.1016/j.jcyt.2017.12.012 |issn=1465-3249 |pmid=29397307|url-access=subscription }}</ref><ref>{{Cite journal |last1=Bozorgmehr |first1=M. |last2=Gurung |first2=S. |last3=Darzi |first3=S. |last4=Nikoo |first4=S. |last5=Kazemnejad |first5=S. |last6=Zarnani |first6=A.-H. |last7=Gargett |first7=C.E. |date=2020 |title=Endometrial and Menstrual Blood Mesenchymal Stem/Stromal Cells: Biological Properties and Clinical Application |journal=Frontiers in Cell and Developmental Biology |language=English |volume=8 |page=497 |doi=10.3389/fcell.2020.00497 |doi-access=free |issn=2296-634X |pmc=7364758 |pmid=32742977}}</ref>

==Endometrial receptivity==

Endometrial receptivity is a crucial factor in achieving successful embryo implantation in assisted reproduction treatments. It refers to the ability of the endometrium to accept an embryo during a specific time window, known as the "implantation window." The synchronization between endometrial development and the embryo is essential to ensure a successful pregnancy.

Currently, there are three main tests that help evaluate endometrial receptivity and optimize fertility treatments:

'''ERA (Endometrial Receptivity Analysis):''' 

This genetic test analyzes the expression of specific genes in the endometrium to identify whether it is in a receptive, pre-receptive or post-receptive phase, allowing the ideal moment for embryo transfer to be personalized.

'''EMMA (Endometrial Microbiome Metagenomic Analysis)''' and '''ALICE (Analysis of Infectious Chronic Endometritis):''' 

Perform a test on the intrauterine microflora, using a small sample of the endometrium, in order to determine the presence of microorganisms that may promote or harm embryo implantation.

==Additional images==
<gallery>
 Image:Endometrial adenocarcinoma (1).jpg|Endometrioid adenocarcinoma from biopsy. H&E stain.
 Image:Endometrium_ocp_use2.jpg|Micrograph of decidualized endometrium due to exogenous [[progesterone]]. [[H&E stain]].
 Image:Endometrium_ocp_use1.jpg|Micrograph of decidualized endometrium due to exogenous progesterone. H&E stain.
 Image:Endometrial stromal condensation high mag.jpg|Micrograph showing endometrial stromal condensation, a finding seen in [[menses]].
</gallery>

==See also==
* [[CYTL1]], also known as cytokine-like like protein 1.
* [[Endometrial ablation]]
* [[List of distinct cell types in the adult human body]]

==References==
{{Reflist}}

==External links==
* {{SUNYAnatomyFigs|43|05|15}} - "The uterus, uterine tubes and ovary with associated structures."
* {{BUHistology|18902loa}} - "Female Reproductive System uterus, endometrium"
* {{EmbryologySwiss|gnidation/role02}}
* {{OklahomaHistology|20_01}}
* [https://web.archive.org/web/20061205114451/http://medlib.med.utah.edu/WebPath/FEMHTML/FEM017.html Histology at utah.edu. Slide is proliferative phase - click forward to see secretory phase]

{{Female reproductive system}}
{{Authority control}}

[[Category:Mammal female reproductive system]]
[[Category:Pelvis]]